The present invention relates to an objective lens actuator to be used in an optical disc apparatus for reproducing or recording information onto or from a recording surface of an optical disc.
In an optical disc apparatus for recording information onto a disc-like information recording medium or reproducing information recording thereon, there is mounted an objective lens actuator for actuating or driving an objective lens in a focusing direction and a tracking direction, so as to record or reproduce the information, correctly, following a tack on the optical disc.
In general, such the objective lens actuator is built up with, a magnetic circuit made of a yoke and a permanent magnet, a moving part attached with an objective lens thereon, a holder for holding this moving part thereon, and a support member for elastically supporting the moving part with respect to the holder. Onto the moving part are attached a focusing coil and a tracking coil, wherein the moving part is actuated or driven in the focusing direction through an electromagnetic force generated due to an effect of flux from the permanent magnet attached on a yoke when supplying current into the focusing coil. In the similar manner, the moving part is driven in the tracking direction through an electromagnetic force generated due to the effect of flux from the permanent magnet attached on the yoke when supplying current into the tracking coil.
The density of magnetic flux effecting upon the coil is almost uniform at a central portion of the permanent magnet, but it becomes small, gradually, in a peripheral portion of the permanent magnet. If the moving part is operated or actuated to locate in such an area where the magnetic flux density is not uniform, then a change is caused in a magnitude of the electromagnetic force generating on the coil, then a moment of rotation is produced around a center of gravity of the moving part.
Explanation will be given on this principle, by referring to FIG. 10 for showing the typical structures of the tracking coils 737r and 737l and the permanent magnet 736c of the objective lens actuator.
FIG. 10 is a side view for showing the force, which is generated on the tracking coil, and also the moment around an “x”-axis due to that force, when the moving part is actuated in a “z” direction, within the objective lens actuator of the conventional art.
As is shown in FIG. 10, when supplying current into the tracking coils 737r and 737l under the condition that the moving part is actuated by “Δz” into the “z” direction, an unbalance is generated in the magnetic flux density between the position on upper sides of the tracking coils 737r and 737l and lower sides thereof, and there are remaining the magnetic forces, respectively, i.e., (Ftl3-Ftl2) downwards in the “z” direction on the tracking coil 737l and (Ftr3-Ftr2) upwards in the “z” direction on the tracking coil 737r. As a result thereof, the moment “M” acts onto the moving part, rotating around the “x”-axis.
This rotation moment comes into a cause of reason of tilt of the objective lens. Since the tilt of the objective lens results into generation of an aberration, and thereby deterioration of recording quality and a reproduced signal, therefore it is necessary to suppress the tilt of the objective lens.
For such the problem of the tilt of the objective lens, there is already known an objective lens actuator for acting on the tracking coils and/or the focusing coil, wherein the magnet (hereinafter, being called, “a permanent magnet”) is determined to be in such a configuration that the moments due to the electromagnetic force to tilt the objective lens substantially offset or cancel each other (i.e., an example shown in the following Patent Document 1). With this, the moment rotating around the center of gravity of the moving part, which is produced due to the electromagnetic force generated in the tracking coils, and the moment rotating around the center of gravity of the moving part, which is produced due to the electromagnetic force generated in the focusing coil, are cancelled with each other, and thereby preventing or protecting the objective lens from the tilt thereof.
[Patent Document 1] Japanese Patent Laying-Open No. 2001-101687 (2001) (see claim 1 and FIGS. 1 to 5).